Various kinds of soldering fluxes and solder paste compositions have conventionally been used for solder joining of electronic circuit components, or the like. Particularly, the flux removes metal oxide on solder and on a substrate surface, and also prevents reoxidation of metal during soldering. Hence, the flux reduces surface tension of the solder and is therefore indispensable to satisfactorily perform the soldering.
However, the conventional flux and solder paste composition may cause the problem that cracks occur in a flux residue after the soldering and water enters the cracks, thus producing a short-circuit defect between component leads. There is a high probability that this problem occurs particularly on an on-vehicle substrate subjected to a large temperature difference and large vibration during use.
The method for applying solder paste can be roughly classified into printing method and discharge method. The printing method is one in which a metal mask, a silk screen, or the like, each having holes disposed in soldering portions, is mounted on a printed circuit board, and solder paste is applied from thereabove. The discharge method is one in which solder paste is applied to the soldering portions one by one by using a dispenser or the like. There arises a disadvantage that the application is not performable on a fine-pitch pattern by the discharge method. For example, the printing method is employed in solder joining of electronic circuit components or the like to a fine-pitch circuit board.
Mounting technology is highly densified with downsizing electronic devices, and there is an increasing trend towards a finer pitch. Therefore, the solder paste is required to have excellent printability (transfer performance) in addition to conventionally required characteristics (stability, reliability, and the like). For example, when using the metal mask, the printability is to efficiently transfer to the substrate the solder paste adhered to a wall surface or the like of an opening in the metal mask. Several means, such as miniaturization of metal particle size, and increasing the amount of wax, have been proposed so far in order to improve the printability. Although the miniaturization of metal particle size ensures improving the printability, “storage stability” and “wettability” are poor. The increasing the amount of wax makes it difficult to adjust viscosity and the wettability is apt to be poor.
The present applicant has found so far that a thermoplastic acrylic resin having glass transfer temperature of less than −50° C. is used as a base resin in order to suppress cracks of the flux residue, and that an active action can be promoted when the acid value of the thermoplastic acrylic resin is 50 mg KOH/g or more (Patent Document 1).
Patent Document 2 describes that a resin having acid value of 50 mg KOH/g or less is added to solder paste in order to improve washability against the flux residue after soldering, that the resin is rosin obtained by purifying a natural resinpine tree gum, and that a resin having a high acid value is mixed in order to compensate for the active action of the resin having a low acid value. However, there is no description of the necessity for improving the printability, and there is no description of the use of a resin other than the rosin. The flux residue is to be removed by washing, and hence it is difficult to improve the crack resistance of the flux residue.